Recording EEG signals

This set covers the fundamental terminology, hardware functions, and signal processing concepts involved in recording EEG signals, including amplifiers, filters, and digital conversion.

Differential Amplifiers

Devices that measure the difference in voltage (potential) between two points on the scalp rather than absolute values.

Common Mode Rejection (CMR)

A feature of differential amplifiers where any activity appearing in phase and with the same amplitude at both inputs is rejected, resulting in a flat line output.

Sensitivity

The ratio of input voltage to the signal deflection produced, expressed in $\mu V/mm$. A higher sensitivity setting produces a smaller output signal.

Gain

The ratio of signal voltage at the input to the signal voltage at the output; higher gain results in a larger output signal.

Alpha

An EEG frequency range of $8-13\,Hz$.

Beta

An EEG frequency range of $>13\,Hz$.

Theta

An EEG frequency range of $4-8\,Hz$.

Delta

An EEG frequency range of $<4\,Hz$.

Low Frequency Filter (LFF)

Also known as a high pass filter, it is a device used to attenuate unwanted slow frequency artifacts.

High Frequency Filter (HFF)

Also known as a low pass filter, it is a device used to attenuate unwanted high frequencies such as EMG.

Notch Filter

Also known as a band reject filter, it is specifically used to attenuate AC interference at $50\,Hz$.

Time Constant (TC)

Describes the effect of LFFs on a square wave pulse, defined as the time taken for the amplitude to fall to $37\%$ (attenuation of $63\%$) of its original value.

Roll-off

A term referring to the specific shape of the frequency-attenuation curve.

Sampling Rate

Determines the accuracy of analog to digital conversion, measured in $Hz$ or samples per second.

Nyquist Frequency

The principle stating that the sampling rate must be at least twice the fastest frequency in the waveform.

Aliasing

A phenomenon or distortion that occurs during analog to digital conversion if the sampling rate is insufficient.

Montage

The organized combination of electrode inputs to form various channels of output on an EEG recording.